This invention relates to a sensor node for carrying out an intermittent operation, a base station for receiving data from the sensor node, and a sensor network system.
Recently, a network system (hereinafter, referred to as sensor net) has been studied, in which a small electronic circuit having a radio-communication function is added to a sensor to introduce various pieces of information in a real world into an information processing apparatus in real time. A wide range of applications have been considered for the sensor net. For example, there is a medical application, in which biological information such as a pulsebeat is always monitored by a small electronic circuit with a radio circuit, a processor, a sensor, and a battery integrated thereon, monitored results are sent to a diagnosis apparatus through radio-communication, and a user's health condition is determined based on the monitored results (“Artifact-Resistant Power-Efficient Design of Finger-Ring Plenthysmographic Sensors”, IEEE Transactions On Biomedical Engineering, Vol. 48, No. 7, July 2001, pp. 795-805 by Sokwoo Rhee et al.).
In order to put the sensor net into practical use widely, it is important to keep an electronic circuit (hereinafter, referred to as sensor node) on which a radio-communication function, a sensor, and a power source such as a battery are mounted without maintenance for a long period of time, to allow the electronic circuit to continue to transmit sensor data, and also important to miniaturize the outer shape of the electronic circuit. Therefore, an ultra-small sensor node capable of being set anywhere is being developed. In this stage, in terms of a practical application, it is considered to be necessary that a sensor node can be used without exchanging a battery for about one year from both aspects of maintenance cost and ease of use.
For example, “Smarter Sensors In Silicon” [online] [searched on 16 Feb. 2004], the Internet, URL: http://www.xbox.com/Support/Support_pdf_files/Motetraining/Hardware.pdf> introduces a prototype of a small sensor node having a diameter of about 3 cm, which is referred to as [Mica2 Dot]. This Mica2 Dot is provided with: an RF chip where functions necessary for a wireless communication are integrated; and a processor chip that is low in electric power consumption. In this prototype, with the intermittent operation where it is in a waiting state of 99% of the time, it is intermittently actuated for only the remaining 1% of the time, a sensor is moved, and a result is wirelessly communicated, a small battery can be used to carry out the operation of about one year.
A sensor net requires two kinds of devices, the small sensor node for carrying out the wireless communication as mentioned above and a device (hereafter, referred to as a base station) for wirelessly collecting sensed data and connecting to a wired network such as the Internet. The sensor node is driven by a battery in many cases, in view of its small size and mobility. On the contrary, the base station is stationary and driven by an AC power source, in many cases.
Because of those reasons, there are several functional differences between the sensor node and the base station. For example, in the base station, the mechanism for making the performance of hardware higher and improving a reception performance is established, in order to improve the communication performance, namely, the communication reliability and the communication distance. For example, a space diversity for setting a plurality of antennas, comparing and analyzing data obtained from both of the antennas and then creating an original normal data is typical. Moreover, the application of the space diversity such as an angle diversity or a polarization wave, and a frequency diversity of using two kinds of different frequencies are listed. For this reason, different hardware configurations are typically used between the sensor node and the base station.
Also, as a method of protecting a traffic quantity from being increased in a network, a method of installing a cache region to transiently store data having a high access frequency is known (for example, refer to JP 11-65915 A).